This invention relates to a burner for producing glass fine particles used in fabricating a porous glass base material for a communication or an optical system.
Optical fibers, light guides, image fibers or rod lenses used in the fields of communication and optical systems are fabricated by producing a predetermined base material by means such as MCVD method, VAD method of OVD method, spinning the optical fiber base material thus obtained, and reducing in diameter a rod lens base material.
A conventional method of synthesizing a porous glass base material at high speed for an optical fiber by a VAD method of the abovementioned methods is alredy disclosed as a double flame burner system (Report No. 367 of National Conference of Semiconductor Material Section in the Association of the Japanese Electric Communication Society in 1983, and Report No. 1138 of National Conference of the Association of the Japanese Electric Communication Society in 1984).
The outline of this double flame burner system is as shown in FIG. 6, and will be briefly described.
In FIG. 6, a burner 1 made of a multiwall tube structure has an inner flame generator 2, and an outer flame generator 4 provided on the outer periphery of the generator 2 through a sealing gas passage 3 in a relative relationship that the end of the generator 2 is disposed inside the inner end of the generator 4.
The generator 2 of the both generators is formed of four-wall passage, and the generator 4 is formed of five-wall passage in such a manner that the passages are concentrically disposed.
1.54 liter/min. of SiCl.sub.4, 0.41 liter/min. of GeCl.sub.4, 10.5 liter/min. of H.sub.2, 5 liters/min. of Ar and 15 liters/min. of O.sub.2 are supplied to the generator 2 by the burner 1 as an example of the VAD method, 5 liters/min. of Ar is supplied to the sealing gas passage 3, 5 liters/min. of Ar, 0.41 liter/min. of SiCl.sub.4, 24 liters/min. of H.sub.2, 5 liters/min. of Ar and 25 liters/min. of O.sub.2 are supplied to the generator 4, the flame hydrolytic reaction products of the respective gases, i.e., soot-state glass fine particles are accumulated through the burner 1 in a desired shape to form a porous glass base material 5.
When the base material 5 is thus formed, an inner flame having a length l.sub.1 is generated from the end of the generator 2 as shown in FIG. 6, an outer flame having a length l.sub.2 is generated from the end of the generator 4, and, since the flames continue in the longitudinal direction, the total flame length L of the burner 1 becomes L=l.sub.1 +l.sub.2.
The total flame is considerably longer than the single flame, and a gas stream of the raw material system shown by hatched lines in FIG. 6 stays long in the flame.
As a result, the growth of glass fine particles in the flame is accelerated, the particle diameter increases, and the accumulating efficiency is enhanced by the inertial effect to synthesize the porous glass base material 5 at a high speed.
In the case of the abovementioned double flame burner system, it is adapted for high speed synthesis of the porous glass base material 5, but when doping raw material for forming a refractive index distribution is supplied to the generator 2 to react with the flame, the dopant is diffused more than required in the flame due to the long staying time in the flame, and the density distribution is broadened (flattened).
Thus, the refractive index of the porous glass base material 5 is formed in an (step index) shape as shown in FIG. 7, and the porous glass base material for the optical fiber having GI type refractive index distribution and triangular refractive index distribution cannot be obtained.
In addition, when the doped raw material such as GeCl.sub.4 stays in the flame for long time, crystalline GeO.sub.2 is generated in the flame, bonded to the base base material 5, and bubbles are formed in the base material at transparent vitrifying time.